Clerics are good vs anything tbh.
But yeah, Clerics are exceptional at fighting incorporeal demons. And I have 2 spells that will wreck any incorporeal demon's day real fast: and .
Both of these spells are affected by SR and demons usually have respectable SR, so I would use items that can buff your caster level for max spell pen, such as Dweomer's Essence, prior to casting either of these.
Dismissal can also work if you don't have Dispel Evil yet. But let's pretend that levels aren't important right now.
Depending on how much time you have to prep before this fight, you can use Hallow if you have 24 hours, or Magic Circle against Evil if you're pressed for time, and Planar Refuge/Symbol of Sealing/Forbiddance if you're only "mostly" pressed for time (1 hour, 10 minute, and 6 round cast times, respectively). There may be certain situations where you have no prep time at all because you get ambushed (I would still take the time to set up a Magic Circle against Evil in this case), and there may be situations where "the demons come once a week or once a month". So if you have time, I would ward your intended combat zone with as many of these spells as possible.
If you're facing a demon and you're on his plane of existence, or any plane that isn't the Material Plane, I would most definitely take an hour to set up Planar Refuge whenever possible-- this circumvents a lot of potential DM shenanigans, as well as environmental hazards; such as if this plane has no air and you're walking around holding Bottles of Air in one hand while you're searching for wherever this demon is going to pop up, it's a pain in the butt to fight one-handed or having to sovereign glue that Bottle of Air to your face, so Planar Refuge pwns whenever you're fighting a demon and you're not on the Material Plane and you have at least 1 hour before he arrives. If you have 24 hours or more before the demon shows up, and you're able to get Hallow active, you can put an additional spell effect into it, and there's a litany of spells that you can choose to imbue the Hallow with, like aid, bane, bless, daylight, death ward, detect evil, detect magic, dimensional anchor, discern lies, dispel magic, endure elements, freedom of movement, invisibility purge, protection from energy, remove fear, resist energy, silence, tongues, or zone of truth.
Dimensional Anchor is HIGHLY useful for preventing any teleport/plane shifting shenanigans.
Demons typically have decent-to-high DR/cold iron and DR/good and their SR is usually also high, so hand out Communal Align Weapon (Good) for your allies' weapons to bypass the demons' DR/good and this is going to be a major source of your party's overall damage. Your Channel Energy deals full damage to incorporeal, and Daylight can certainly piss in the corn flakes of any Shadow Demon's hearty breakfast. Spear of Purity is a cheapy-cheap nuke spell that is really effective. Spell Resistance and Spell Immunity are certainly helpful. I would probably put Spell Immunity vs. Dispel Magic on the ally who is most-reliant upon keeping buffs active. If you don't have a Cold Iron/Ghost Touch weapon, you can use Instant Weapon to create a weapon of Force that harms incorporeal, or Chain of Perdition to create a 10ft chain of Force that you can use to perform some Combat Maneuvers. With Demons' high SR, Chain of Perdition would probably only be useful if you're surrounded by a pack of lower level demon minions, and Chain of Perdition isn't so great that I'd waste Dweomer's Essence on it.
Then there are the usual buffs that are always good, like Protection from Energy, Resist Energy, and Bless. If you have the Isolation Subdomain, you can use Wall of Force to set up chokepoints or prevent them from coming through walls, floors, and ceilings. If you don't have access to Wall of Force, then do it with Symbol of Sealing.
After the fight is over, Clerics are still fantastic because of Cure spells, Restoration spells, and Resurrection spells.
When it comes to offensive magic clerics are usually less effective than arcane caster, except vs two things. Those things are undead and outsiders, particularly evil outsiders. While some of the spells will do full damage vs an incorporeal creature, some do not. But an ectoplasmic spell only raises the spell one slot. Since a lot of the cleric spells are highly effective vs evil outsiders this still makes them very effective even after applying the metamagic feat.
Spear of Purity is a 2nd level spell that is a ranged touch attack doing 1d6 per level and blinds evil outsiders. So, when cast normally still doing 1d6 per 2 levels. As an ectoplasmic spell it would be a 3rd level spells doing full damage. Holy Smite is basically an area of effect version of the spell.
Spiritual Weapon is also highly effective vs incorporeal creatures. Clerics have a medium BAB so you will eventually get multiple attacks and it automatically continues to attack, unless you need to redirect it. This means you can just cast the spell and then do something else. Against a really tough incorporeal creature you can spam this spell out and get an absurd number of attacks. What else is a higher-level caster going to do with spell levels that are no longer that useful?